Typically, light is manipulated using glass lenses, but this approach has limitations. Exposure to high-energy laser beams heats up the glass which can lead to damage, or at least an unwanted modification of the refractive index. There are also losses at the boundaries of different components in an optical setup. One way to mitigate these effects is to use gases to control light. Depending on the density of molecules, the refractive index of a gas varies and thereby alters the optical path of light: in nature, this manifests as a mirage. For example, when driving on a hot day, the road can appear to have pools of water (pictured). This is because the air immediately above the hot tarmac is less dense than the cooler air above — causing light to refract, such that the sky is reflected in the road (as if there is a puddle of water). However, using gases to control light comes with its own technical challenges — the refractive indices of gases are relatively low and controlling gases is difficult. Now, writing in Nature Photonics, Yannick Shrödel and colleagues use ambient air to deflect the path of high-power laser beams.
The experiment was then repeated using ultrashort laser pulses, with a peak power of 20 GW. The beam quality was preserved, and the peak power only slightly reduced after diffraction. Compared to using solid media for diffraction, this result using ambient air shows a peak power increased by orders of magnitude (previous work with quartz showed a peak power of 0.5 MW). The peak power is limited by the critical power of air (the value at which self-focusing occurs: a non-linear optical process triggered by exposure to intense radiation). Using gases lighter than air, such as helium, would have lower refractive indices (and higher critical power), which would enable the diffraction of laser beams with power in the terawatt range, useful for manufacturing applications as well as plasma physics.
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